Storage system and data migration method of storage system

ABSTRACT

The present invention suppresses the generation of redundant I/O and improves the response to the host during data migration. When migrating data from the migration source volume to the migration destination volume, the access destination of the host is switched to the second storage device. When data requested by the host has not yet been migrated, data is read from the first storage device. When the frequency of access to the non-migrated area reaches a prescribed value, the target IP address is changed and discovery is implemented so as to switch the access destination of the host to the first storage device. When the frequency of access to the migrated area in the first storage device reaches a prescribed value, the access destination of the host is switched to the second storage device. The access destination of the host is controlled based on the data migration state and access state, and the generation of redundant I/O can be suppressed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to and claims priority from Japanese PatentApplication No. 2004-334264 filed on Nov. 18, 2004, the entiredisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a storage system and a data migrationmethod for a storage system.

2. Description of the Related Art

A storage system uses a memory device such as a hard disk drive andcreates a memory area based on RAID (Redundant Array of IndependentDisks). This physical memory area (logical volume), for example, isprovided to a host computer (hereinafter referred to as a host) such asa server machine. The application program operating on the host providesinformation processing service to the client terminal connected to thehost by writing data in the logical volume or reading data from thelogical volume.

The data volume to be stored in the storage system is increasing day byday, and, depending on the type of data, there are cases where it ismandatory to store such data for a long period of time. Thus, in orderto deal with the increasing demand, a new, high-capacity storage deviceis added.

When a high-capacity storage device is added to the storage system, datastored in the old, low-capacity storage device is migrated to the new,high-capacity storage device. As a method of migrating data from acertain storage device to another storage device, a method has beenproposed for migrating data in a so-called online state withoutdisrupting the service to the host (Japanese Patent Laid-OpenPublication No. 2000-187608).

With a conventional storage system, the access destination of the hostis switched to the storage device of the migration destination, and datamigration is implemented between the storage devices. During datamigration, the host accesses only the storage device of the migrationdestination to read and write the intended data. When data requested tobe read from the host has already been copied to the storage device ofthe migration destination, the storage device of the migrationdestination will provide the data that it stores to the host.

Meanwhile, when data requested to be read from the host has not yet beencopied to the storage device of the migration destination, the storagedevice of the migration destination read data from the storage device ofthe migration source and copies this in its own volume, and provides theacquired data to the host.

Therefore, when access is made to data not yet copied to the storagedevice of the migration destination, I/O (Input/Output) will beperformed twice; that is, (i) data copy from the storage device of themigration source to the storage device of the migration destination, and(ii) data transmission from the storage device of the migrationdestination to the host. Thus, much time is required for providing thenon-migrated data to the host, and the response of the storage systemwill deteriorate. Further, when the data migration between storagedevices and the data communication between the storage device of themigration destination and the host respectively use the samecommunication network, wasteful data will flow on the network due to theI/O operation being conducted twice, and the network traffic willincrease.

In particular, for instance, when the host is to distribute continuousdata such as video data, the influence resulting from the generation ofwasteful I/O operation will be significant. When a series of data groupsis to be read out as with video data, if this series of data groups hasnot been copied to the storage device of the migration destination, theforegoing two I/O operations will be performed continuously. Therefore,the response of the storage device will deteriorate, the response of theservice to be provided from the host to the client terminal will alsodeteriorate, and the network traffic will also increase.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a storagesystem and a data migration system of a storage system capable ofreducing wasteful data transfer by controlling the access destination ofa host device based on the data migration state between storage devices.Another object of the present invention is to provide a storage systemand a data migration system of a storage system capable of preventingthe generation of a plurality of I/O operations regarding the same databy switching the access destination of the host device to the storagedevice of the migration source when access to data not yet copied to thestorage device of the migration destination occurs frequently. Stillanother object of the present invention is to provide a storage systemand a data migration system of a storage system capable of suppressingthe generation of wasteful I/O operations and reducing the networktraffic when the data migration between storage devices and the datacommunication between the host device and storage device use the samecommunication network. Other objects of the present invention willbecome clear from the description of the embodiments provided below.

In order to achieve the foregoing objects, the storage system of thepresent invention is a storage system in which a first storage device, asecond storage device and a host device are mutually connectedcommunicably, comprising: a data migration unit for migrating datastored in a first volume of the first storage device to a second volumeof the second storage device; a data migration state management unit formanaging the migration state of data from the first volume to the secondvolume; a first access request processing unit for processing the accessrequest from the host device to the second volume; and a first switchingindication unit for switching the access destination of the host deviceto the first storage device based on the access state to thenon-migrated area in which data has not yet been migrated from the firstvolume to the second volume.

For instance, the first storage device, second storage device and hostdevice may be mutually connected via an iSCSI (internet Small ComputerSystem Interface) network. The first switching indication unit may beprovided to the second storage device, and the second switchingindication unit may be provided to the first storage device,respectively.

And, the first access request processing unit is capable of processingthe access request from the host device while the data migration unit ismigrating data from the first volume to the second volume. Thereby, datacan be migrated without disrupting the service to the host device.

The first access request processing unit acquires data from the secondvolume and provides this to the host device when the access request fromthe host device is an access request to a migrated area in which datahas been migrated to the second volume. Meanwhile, [the first accessrequest processing unit] acquires data from the first volume andprovides this to the host device when the access request from the hostdevice is an access request to the non-migrated area.

Nevertheless, the first switching indication unit switches the accessdestination of the host device to the first storage device when thefrequency of access from the host device to the non-migrated areareaches a first prescribed value. Thereby, for instance, in a case wheresequential reading is required, by switching the access destination ofthe host device to the first storage device storing such data, thegeneration of wasteful I/O operations regarding non-migrated data canalso be prevented.

A second access request processing unit for processing the accessrequest from the host device to the first volume; and a second switchingindication unit for switching the access destination of the host devicefrom the first storage device to the second storage device based on theaccess state to the migrated area in which data has been transferred tothe second volume among the access requests from the host device to thefirst volume may also be provided. In other words, in a case where theaccess destination of the high-level device is switched from the secondstorage device to the first storage device due to the frequentoccurrence of the access request to the non-migrated area, when the hostdevice frequently accesses the migrated area, the access destination ofthe host device is reswitched to the second storage device. Thereby, thegeneration of wasteful I/O operations regarding migrated data can alsobe suppressed.

Further, after the data migration unit completes the data migration, thesecond switching indication unit confirms whether the access destinationof the host device is the first storage device or the second storagedevice, and switches the access destination of the host device to thesecond storage device when the access destination of the host device isthe first storage device.

The first switching indication unit may also switch the accessdestination of the host device from the second storage device to thefirst storage device by copying the second constitution information ofthe second storage device to the first storage device, rewriting thefirst constitution information of the first storage device with thesecond constitution information, and providing a trigger signal forurging the redetection of the access destination to the host device.

For example, a storage management device for transmitting the triggersignal to the host device may also be provided, wherein the storagemanagement device transmits the trigger signal to the host device bynotifying the storage management device to the effect that the firstconstitution information of the first storage device has been rewrittenwith the second constitution information. Or, the trigger signal may beprovided to the host device by logically disconnecting the connection ofthe second storage device and the host device.

For example, in the case of a storage system using iSCSI, a name servermay be used as the storage management device. A name server associatesand manages the target name (iSCSI name) and IP address (iSCSI address)of the respective nodes participating in the storage system. Here, thetarget name and IP address can be separated, and, as a result, even whenthe position within the network is changed, the host device is able touniquely specify the target storage device by performing discovery.Discovery is processing for detecting a device connected to the network.

The host device makes an inquiry to the name server so as to detect thetarget storage device. Here, the second storage device is succeeding thefirst constitution information (IP address and soon) of the firststorage device, and this succession is notified to the name server.Therefore, the name server notifies the host device to the effect thatthe IP address of the iSCSI target has been changed, and the host deviceswitches the access destination based on such notification. The schemeof this switching can be employed in both cases of switching from thefirst storage device to the second storage device, and of switching fromthe second storage device to the first storage device.

Further, for example, the first storage device, the second storagedevice and the host device may be mutually connected in a two-waycommunicable manner via a switch; and the switch may be provided with atleast the data migration state management unit and the first switchingindication unit. As the switch, for example, a gigabyte-level Ethernet(registered trademark) switch may be used. Moreover, the secondswitching indication unit may be provided to the switch.

The data migration method for a storage system according to a differentperspective of the present invention is a data migration method ofmigrating data from the first storage device to the second storagedevice in a storage system in which a first storage device, a secondstorage device and a host device are mutually connected communicably,comprising: a first switching step of switching the access destinationof the host device from the first storage device to the second storagedevice; a data migration step of migrating data stored in a first volumeof the first storage device to a second volume of the second storagedevice; a management step of managing the data migration state of datafrom the first volume to the second volume; an access request processingstep of processing the access request from the host device to the secondvolume; a step of detecting the access state to the non-migrated area inwhich data has not yet been migrated from the first volume to the secondvolume; and a second switching step of switching the access destinationof the host device from the second storage device to [the] first storagedevice based on the access state to the detected non-migrated area.

Further, a third switching step is provided after the second switchingstep, wherein the third switching step reswitches the access destinationof the host device from the first storage device to the second storagedevice based on the access state to the migrated area in which data hasbeen transferred to the second volume among the access requests from thehost device to the first volume.

Moreover, a fourth switching step is provided after the third switchingstep, wherein, when the data migration is complete, the fourth switchingstep confirms whether the access destination of the host device is thefirst storage device or the second storage device, and switches theaccess destination of the host device to the second storage device whenthe access destination of the host device is the first storage device.

The functions, means and steps of the present invention, whether inwhole or in part, may be constituted as a computer program to beexecuted, for example, with a microcomputer. And, this computer programcan be fixed in a storage medium such as a hard disk,. optical disk orsemiconductor memory to be distributed. Or, the computer program may bedistributed via a communication network such as the Internet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing the concept of an embodiment ofthe present invention;

FIG. 2 is an explanatory diagram showing a frame format of the overallconstitution of the storage system pertaining to the first embodiment;

FIG. 3 is a block diagram showing a schematic constitution of thestorage system;

FIG. 4 is an explanatory diagram showing an example of the configurationinformation;

FIG. 5 is an explanatory diagram showing an example of the storagemanagement table;

FIG. 6 is an explanatory diagram showing an example of the datamigration state management table;

FIG. 7 is an explanatory diagram showing the protocol hierarchy ofiSCSI;

FIG. 8 is an explanatory diagram showing the data structure of iSCSI;

FIG. 9 is a sequence flowchart showing the overall data migration;

FIG. 10 is a flowchart showing the read processing to a non-migratedarea;

FIG. 11 is a flowchart showing the write processing;

FIG. 12 is a sequence flowchart showing the state of switching the pathto the storage device of the migration source based on the access stateto the non-migrated area;

FIG. 13 is a flowchart showing the read processing to the migrated area;

FIG. 14 is a flowchart of the path switching processing to beimplemented after the completion of data migration;

FIG. 15 is a block diagram showing the schematic constitution of thestorage system of the second embodiment;

FIG. 16 is a sequence flowchart showing the overall data migration;

FIG. 17 is a sequence flowchart showing the state of switching the pathfrom the storage device of the migration destination to the storagedevice of the migration source;

FIG. 18 is an explanatory diagram showing a frame format of the storagesystem of the third embodiment; and

FIG. 19 is a block diagram showing a schematic of the storage device ofthe fourth embodiment.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Embodiments of the present invention are now explained with reference tothe drawings. FIG. 1 is an overall conceptual diagram of the presentembodiment. Although described in detail later, as shown in FIG. 1, thestorage system of the present embodiment has a first storage device 1, asecond storage device 2, a host 3, and a storage management device 4,and, for example, the respective devices 1 to 4 are mutually connectedvia a communication network employing TCP/IP (Transmission ControlProtocol/Internet Protocol) of the Internet or the like.

The first storage device 1 is the storage device of the data migrationsource, and, for instance, may comprise a migration source volume 1A,and a control unit 1B. Further, the control unit 1B may include a datamigration unit 1C and a second switching indication unit 1D.

The second storage device 2 is the storage device of the data migrationdestination, and, for instance, may comprise a migration destinationvolume 2A, a data migration unit 2B, a data migration state managementunit 2C, an access processing unit 2D, and a first switching indicationunit 2E. Among the above, 2B to 2E can be realized as a control functionof the second storage device 2.

The host 3, for example, is a computer system constituted as a servermachine, and is capable of accessing either the first storage device 1or the second storage device 2 via a communication network. The host 3provides information processing service such as video distribution to aclient terminal not shown.

The storage management device 4 is a computer system for managing thetarget name (iSCSI name) and IP address (iSCSI address) of therespective storage devices 1, 2 participating in the storage system. Thepresent embodiment illustrates an example of employing iSCSI.

When copying and migrating data retained in the first storage device 1to the second storage device 2, the access destination of the host 3 isswitched from the first storage device 1 to the second storage device 2(S1).

For example, the constitution information is notified from the firststorage device 1 to the second storage device 2, the IP address of thesecond storage device 2 is associated with the iSCSI target (targetname), which is the access destination of the host 3, and the change ofthis IP address is notified to the storage management device 4. Thereby,the storage management device 4 will notify the host 3 to the effectthat the IP address of the target has been changed, and the host 3, uponreceiving such notification, will commence discovery. As a result ofthis discovery, the host 3 detects the target (second storage device 2),and switches the access destination to the second storage device 2.

After switching the access destination of the host 3 to the secondstorage device 2, data migration from the first storage device 1 to thesecond storage device 2 is commenced (S2). This data migration, forexample, may be performed in two ways. One method is for the datamigration unit 1C of the first storage device 1 to read data of themigration source volume 1A in a prescribed order, and copying this tothe migration destination volume 2A of the second storage device 2. Theother method is for the data migration unit 2B of the second storagedevice 2 to read data of the migration source volume 1A, and storingthis in the migration destination volume 2A. Either method may beadopted, and either method is capable of migrating data without havingto go through the host 3.

The migration state of data can be managed with the data migration statemanagement unit 2C. Incidentally, although not shown in the drawings, itis also possible to manage the data migration state within the firststorage device 1. The data migration state management unit 2C managesthe state of which data has been copied to the migration destinationvolume 2A, and which data has not been copied to the migrationdestination volume 2A. For example, the data migration state can bemanaged by associating the address of each data to be migrated and theflag representing that [such data] has been migrated.

Even during the data migration, the host 3 is able to read and writeintended data by accessing the second storage device 2 (S3). The accessprocessing unit 2D processes the read request or write request from thehost 3.

For example, in a case where a read request has been issued from thehost 3, the access processing unit 2D uses the data migration statemanagement unit 2C to determine whether the requested data has beencopied to the migration destination volume 2A. The access processingunit 2D reads data from the migration destination volume 2A andtransmits this to the host 3 when such data has been copied to themigration destination volume 2A.

Contrarily, when the data requested to be read from the host 3 has notbeen copied to the migration destination volume 2A, the accessprocessing unit 2D reads the requested data from the migration sourcevolume 1A, stores such read data in the migration destination volume 2A,and thereafter transmits this to the host 3. Incidentally, the writingof data in the volume (destage processing) may be performed after [suchdata] is transmitted to the host 3. In other words, after storing datain the cache memory (c.f. FIG. 3) of the second storage device 2, thisdata is transmitted to the host 3, and destage processing is performedat a prescribed timing.

Based on the data migration from the first storage device 1 to thesecond storage device 2, the migration source volume 1A and migrationdestination volume 2A can be classified into a migrated area VA1 and anon-migrated area VA2. Regarding which data of which address has beenmigrated or has not been migrated is managed with the data migrationstate management unit 2C.

The first switching indication unit 2E monitors the access frequency ofthe host 3 to the non-migrated area VA2 (S4), and, when access to thenon-migrated area VA2 is made a prescribed number of times or more, [thefirst switching indication unit 2E] notifies this to the first storagedevice 1 (S5). This notification is for rewriting the constitutioninformation, and, thereby, the IP address of the first storage device 1can be associated with the target, which is the access target of thehost 3. The first storage device 1 notifies the storage managementdevice 4 to the effect that the IP address of the target has beenchanged (S6).

The storage management device 4 notifies the host 3 that the IP addressof the target has been changed (S7). This IP address change notificationtriggers the commencement of discovery. The host 3 acquires a new IPaddress from the storage management device 4 for rediscovering thetarget, and switches the access destination to the first storage device1 (S8).

As a result of switching the access destination, subsequent access tothe non-migrated area VA2 will be made to the migration source volume1A. Therefore, when the host 3 continuously accesses data of thenon-migrated area VA2, it possible to prevent the generation of aplurality of I/O operations of this data being transmitted from thefirst storage device 1 to the second storage device 2, and further beingtransmitted from the second storage device 2 to the host 3.

Since the first storage device 1 is retaining all data to be migrated,it is able to independently process the access requests from the host 3.Nevertheless, when the host 3 frequently issues a read request regardingdata of the migrated area VA1, as indicated in S1, the second switchingindication unit 1D reswitches the access destination of the host 3 tothe second storage device 2. As a result, data that has been migrated tothe second storage device 2 can be provided from the second storagedevice 2 to the host 3.

As described above, the access destination of the host 3 is controlledin accordance with the data migration state and access state. And, whenthe data migration is complete, the second switching indication unit 1Dconfirms the access destination of the host 3, and switches the accessdestination of the host 3 to the second storage device 2 when the accessdestination of the host 3 is the first storage device 1. As a result,when the data migration is complete, the host 3 will only access thesecond storage device 2 of the migration destination to read or writeintended data.

As described above, according to the present embodiment, service can beprovided to the host 3 even during data migration, generation ofredundant I/O operations can be suppressed, response to the host 3 canbe improved, and the network traffic can be reduced. Embodiments of thepresent invention are now explained in further detail.

1. First Embodiment

FIG. 2 is an explanatory diagram showing a frame format of the overallconstitution of the storage system. Although the respectiveconstitutions will be described in detail later, this storage system maybe constituted by comprising a host 10, a management device 20, a nameserver 30, a switch 40, a first storage device 100 and a second storagedevice 200. And, these respective devices 10, 20, 30, 100 and 200 aremutually connected via the switch 40, and, for example, enables datacommunication based on TCP/IP. In the present embodiment, illustrated isan example of a storage system employing iSCSI.

The host 10 is the iSCSI initiator, and accesses either the firststorage device 100 or the second storage device 200 to read or writedata. The host 10, for instance, is constituted as a server machine, andprovides information processing service such as video distribution to aclient terminal not shown.

The management device 20 gives various instructions to the first storagedevice 100, as well as acquires and manages the various states of thefirst storage device 100. The management device 20 comprises aconstitution management program 21, and comprehends the constitutionalstatus of the storage device under its control. Incidentally, in thefollowing explanation, although the management device 20 manages thefirst storage device 100, the constitution may also be such that itmanages the first storage device 100 and second storage device 200.Further, a device for managing the second storage device 200 may also beprovided separately.

The name server 30 is a device for uniformly managing the iSCSI targetof the storage system. The name server 30 comprises a name managementprogram 31, and manages the IP address and the like of the respectivestorage devices 100, 200.

The first storage device 100 is a migration source storage device. Thefirst storage device 100 acts as the iSCSI target in relation to thehost 3, and acts as the iSCSI initiator in relation to the secondstorage device 200. In other words, the first storage device 100implements the reading and writing of data stored in the migrationsource volume upon being accessed from the host 3 via the switch 40.Further, the first storage device 100, as the initiator, transmits dataof the migration source volume to the second storage device 200, andcopies this in the migration destination volume. This copy processing isexecuted without going through the host 3.

The second storage device 200 is a migration destination storage device.The second storage device 200, as the iSCSI target, is accessed by thehost 3 or the first storage device 100. The second storage device 200stores the data transmitted from the first storage device 100 in themigration destination volume.

Although explained in detail later, prior to the start of the datamigration, the host 10 accesses the first storage device 100 to read orwrite the intended data. And, upon starting the copy of data from thefirst storage device 100 to the second storage device 200, the IPaddress of the target, which is the access destination, is changed, and,by the name server 30 notifying the host 10 of such change of IPaddress, the access destination of the host 10 is switched from thefirst storage device 100 to the second storage device 200.

FIG. 3 is a block diagram showing a more detailed constitution of thestorage system. The host 10, for example, is constituted as a so-calledopen host. As such open host, for instance, employed may be a servermachine loaded with a versatile OS (Operating System) such as Windows(registered trademark) or UNIX (registered trademark) and which accessesthe storage devices 100, 200 via a relatively versatile communicationprotocol such as FC (Fibre Channel), iSCSI (Internet SCSI), TCP/IP(Transmission Control Protocol/Internet Protocol) or the like. Thestorage system of the present embodiment uses iSCSI to transmit andreceive data.

The host 10, for example, may be constituted by comprising one or aplurality of HBAs (Host Bus Adapters) 11, an input/output (I/O) controlprogram 12, and an application program group (abbreviated as“application group” in the drawings) 13. In FIG. 1, for ease ofexplanation, although only one host 10 is shown, it is also possible toprovide a plurality of hosts 10.

The HBA 11 transmits and receives data based on a prescribed protocol.The I/O control program 12, for instance, is a driver program forcontrolling the input/output of data to be conducted with the HBA 11.The application program group 13, for example, is an email processingprogram or a program such as database management software, motionpicture distribution software, or file system, and respectively providesa prescribed information processing service to the client terminal notshown.

The management device 20 is a device for collecting various types ofinformation of the first storage device 100 via a service processor(SVP) 170 described later, or giving necessary commands to the firststorage device 100. The management device 20 is connected to the SVP 170via a communication network such as a LAN (Local Area Network). Further,the management device 20 may also be connected to the first storagedevice 100 via the switch 40. Incidentally, the management device 20 mayalso manage the constitution of the second storage device 200, or aseparate management device may be provided for the second storage device200.

The management device 20, for example, comprises a web browser-based GUI(Graphical User Interface), and, by logging onto the WWW (World WideWeb) server provided by the SVP 170, performs the collection of varioustypes of information and input of commands. In the present embodiment,for example, the execution of data migration is ordered to the firststorage device 100 via the management device 20.

The first storage device 100, for example, may be constituted bycomprising a plurality of CHAs 110, a plurality of DKAs 120, a cachememory 130, a shared memory 140, a connection control unit 150, a memoryunit 160, and a SVP 170.

A plurality of CHAs 100 may be provided to the first storage device 100.Each CHA 110 is a control package for controlling the data transfer withthe host 10. Each CHA 110 comprises a plurality of communication ports,and is able to individually control the data transfer with a pluralityof hosts 10. Further, a single CHA 110 is able to control the datacommunication with the second storage device 200.

A plurality of DKAs 120 may be provided to the first storage device 100.Each DKA 120 is for respectively controlling the data transfer with thememory unit 160. Each DKA 120, for example, by changing the logicalblock address (LBA) designated from the host 10 to the address of aphysical disk, is able to access the respective disk drives 161 so as toread data or write data.

The cache memory 130 is used for storing the write data written from thehost 10, or the read data read by the host 10. The cache memory 130, forexample, may be constituted from a volatile or non-volatile memory. Whenthe cache memory 130 is to be constituted by including a volatilememory, it is preferable to perform the memory backup with a batterypower source or the like not shown. The cache memory 130, for example,may be constituted from two areas; namely, a read cache area and a writecache area. The write cache area, for example, contains a cache face anda NVS (Non-volatile Storage) face, and is able to subject write data tomultiple storage (redundant storage).

The shared memory (also referred to as a control memory) 140, forexample, maybe constituted from a non-volatile memory, or be constitutedfrom a volatile memory. The shared memory 140, for example, storescontrol information or management information. Information such as theforegoing control information may be multiple-managed with a pluralityof memories 140.

The shared memory 140 and cache memory 130 may be constituted asindependent memory packages, or the cache memory 130 and shared memory140 may be provided in the same memory package. Further, a part of thememory may be used as the cache area, and the other part may be used asthe control area. In other words, the shared memory and cache memory maybe constituted as the same memory or memory group.

The connection control unit 150 is used for mutually connecting each CHA110, each DKA 120, the cache memory 130, and the shared memory 140.Thereby, every CHA 110 and DKA 120 will be able to individually accessthe cache memory 130 and shared memory 140. The connection control unit150, for example, may be constituted with an ultra high-speed crossbarswitch or the like. Incidentally, the CHA 110, DKA 120, cache memory 130and shared memory 140 may be integrated into one or a plurality ofcontrollers.

The memory unit 160 is constituted by comprising a plurality of diskdrives 161. The memory unit 160 may be provided inside the same casetogether with the controller portion of each CHA 110 and each DKA 120,or may be provided inside a case separate from the controller portion.

The memory unit 160, for example, may be constituted to contain amixture of a plurality of types of disk drives 161. As the disk drive161, for example, a FC disk (fiber channel disk) , SCSI (Small ComputerSystem Interface) disk, SATA (Serial AT Attachment) disk and the likemay be used. Incidentally, the type of disk is not limited to the above,and a memory device equivalent to the exemplified disk drives or amemory device that may be developed in the future can also be used.

Here, for example, a single parity group is constituted with aprescribed number of disk drives 161; for instance, a set of 3 or a setof 4, and at least one or more logical volumes may be set on thephysical memory area provided by this parity group. And, this logicalvolume is associated with an LU (Logical Unit) so as to be recognized bythe host 10 as an access target.

Incidentally, it is not necessary for the storage resource to be used bythe first storage device 100 to exist entirely in the first storagedevice 100. The storage resource existing outside the first storagedevice 100 may be incorporated and used as though it is a personalstorage resource.

The SVP 170 is connected to each CHA 110 (and each DKA 120), forinstance, via an internal network such as a LAN. Although the SVP 170 isonly connected to each CHA 110 in the drawing, this may also beconnected to each DKA 120. The SVP 170 collects the various statesinside the first storage device 100, and provides such states to themanagement device 20 without change or upon performing processingthereto.

The second storage device 200 may also comprise the same constitution asthe first storage device 100. The second storage device 200, forexample, may be constituted by comprising each CHA 210, each DKA 220, acache memory 230, a shared memory 240, a connection control unit 250, amemory unit 260 having a plurality of disk drives 261, and an SVP 270.Since each of these components of 210 to 270 corresponds respectively tothe components 110 to 170 described in the first storage device 100,redundant explanations thereof will be omitted.

Nevertheless, the second storage device 200 does not have to be the sameconstitution as the first storage device 100. The first storage device100 and the second storage device 200 may respectively have differentconstitutions.

FIG. 4 is an explanatory diagram showing configuration information D1 asan example of the “configuration information”. Configuration informationD1 includes, for example, the device name, target name (iSCSI name), LUN(Logical Unit Number) of the access target and so on, and is stored inthe shared memory.

As described later, the processing of switching the access destinationof the host 10 is commenced by setting the configuration information ofthe storage device of the switching source in the storage device of theswitching destination. For example, as shown in FIG. 4, when switchingthe access destination of the host 10 from the first storage device 100to the second storage device 200, the configuration information D1 ofthe first storage device 100 is copied to the second storage device 200.As a result, the IP address of the second storage device 200 isassociated with the target name or target LUN, and this change of IPaddress is notified to the name server 30.

Contrarily, when switching the access destination of the host 10 fromthe second storage device 200 to the first storage device 100, theconfiguration information D1 of the second storage device 200 is copiedto the first storage device 100, and the IP address of the first storagedevice 100 is associated with the target name.

FIG. 5 is an explanatory diagram showing a portion of the schematicconstitution of the storage management table T1. The storage managementtable T1 may be provided to the name server 30. As shown in FIG. 5(a),in the initial state of this embodiment, an IP address “address 1” ofthe first storage device 100 is associated with the target, which is theaccess destination of the host 10. In this initial state, the host 10 isaccessing the first storage device 100.

FIG. 5(b) shows a state of the access destination of the host 10 beingswitched to the second storage device 200. The second storage device 200notifies the name server 30 that the IP address of the target has beenchanged to “address 2”. This “address 2” is the IP address of the secondstorage device 200. The name server 30 which received this notification,as shown in FIG. 5(b), rewrites the storage management table T1.Further, the name server 30 notifies the host 10 to the effect that theIP address of the target has been changed.

FIG. 5(c) shows a case of reswitching the access destination of the host10 from the second storage device 200 to the first storage [device] 100.As described above, with the storage system of the present embodiment,the correspondence of the target and IP address is uniformly managedwith the name server 30, and, when the IP address is changed based onthe notification from the target device, this change of IP address isnotified to the host 10, which is the initiator device.

FIG. 6 is an explanatory diagram showing a constitution example of thedata migration state management table T2. This data migration statemanagement table T2 may be provided to the first storage device 100and/or the second storage device 200.

The data migration state management table T2, for example, manages, foreach LUN, whether the data of each logical block address (LBA: LogicalBlock Address) contained in such LUN has been migrated to the secondstorage device 200. In the present embodiment, a migration complete flagis associated to each LBA. The migration complete flag is informationfor showing whether data has been migrated (copied) to the memory areaof the migration destination, and “1” being set represents that [data]has been migrated. Incidentally, the management unit of the datamigration state is not limited to LBA. For instance, a plurality of LBAsmay be integrated as a group, and a migration complete flag may be setfor each group.

FIG. 7 is an explanatory diagram showing the layered structure of iSCSI.In order from the bottom, a physical layer and data link layer, an IPlayer, a TCP layer, an iSCSI layer, a SCSI layer, and a SCSI applicationlayer are layered. In the IP layer, data transfer designating the IPaddress is conducted, and, in the TCP layer, data transfer designatingthe TCP port is conducted. In the iSCSI layer, data transfer designatingthe iSCSI_NAME is conducted, and, in the SCSI layer, data transferdesignating the LU number or LBA (Logical Block Address) is conducted.The iSCSI layer exists between the SCSI layer and TCP layer, and, byhousing the SCSI command and SCSI response in a capsule referred to asiSCSI_PDU, transfers data via TCP connection.

FIG. 8 is an explanatory diagram showing the data structure used iniSCSI. The command output from the SCSI application layer of the host 10via the SCSI layer, as shown in FIG. 8, is a command frame 440 includingcommands and data (or only commands). The command frame 440, forexample, is a 6-byte command frame in which the operation code such asthe write command or read command is included in the top byte.

When this command frame 440 reaches the iSCSI layer, the iSCSI layercoverts the SCSI command frame into iSCSI_PDU (Protocol Data Unit) 430.When this PDU 430 passes through the TCP layer, this is converted into aTCP packet 420, and, when this TCP packet 420 passes through the IPlayer, it becomes an IP packet 410. And, a MAC (Media Access Control)address is added to complete a variable-length MAC frame 400. 1500 bytesof data can be stored in the data field.

The SCSI command frame 440, for example, contains a basic header segment441, an additional header segment 442, and a data segment 443. And,included in the basic header segment 441 may be, for example, anoperation code 441A, a data segment length 441B, a LUN number 441C, aninitiative task tag 441D, and an operation code specification field441E.

With the constitution of the illustrated protocol, as a result of thephysical layers and data link layers performing a session with eachother, the IP layers performing a session with each other, the TCPlayers performing a session with each other, the iSCSI layers performinga session with each other, the SCSI layers performing a session witheach other, and the SCSI application layers performing a session witheach other in order, the I/O request output from the host 10 can berespectively with each of the storage devices 100, 200.

For example, with the session among the physical layers and data linklayers, as a result an ARP (Address Resolution Protocol) request beingmade and a corresponding ARP response being provided, their mutual MACaddresses can be acquired. With the session among the IP layers, as aresult of a ping request being made and a corresponding ping responsebeing provided, it will be possible to mutually confirm whether anopponent (IP address) exists.

With the session among the TCP layers, three packets are exchanged tosynchronize the sequence numbers. With the session among the iSCSIlayers, the iSCSI connection is established with a logon interface forexchanging the logon request and corresponding logon response (forexample, the IP address, iSCSI_NAME of the logon requesting host, andTCP port number are used).

With the session among the SCSI layers, for example, a SCSI command suchas a read command or write command is transmitted from the host 10 toeither the first storage device 100 or second storage device 200. Withthe session among the SCSI application layers, write data is transmittedfrom the host 10 to either the first storage device 100 or secondstorage device 200, or, read data is transmitted from either the storagedevice 100 or 200 to the host 10.

The operation of the present embodiment is now explained with referenceto FIG. 9 to FIG. 14. Foremost, FIG. 9 is a schematic sequence showingthe overall processing of data migration.

For example, a user such as a system administrator indicates the startof migration to the first storage device 100 via the management device20 (S11). The indication of this start of migration, for example, maycontain information or the like for respectively specifying themigration source volume and migration destination volume.

When the execution of migration is designated from the management device20, as described together with FIG. 4, the first storage device 100acquires its own configuration information (S12), logs onto the secondstorage device 200 (S13), and copies this configuration information tothe second storage device 200 (S14).

The second storage device 200 personally sets this configurationinformation, associates the IP address of the second storage device 200with the target name (S15), and notifies the name server 30 to theeffect that the IP address has been changed (S16).

The name server 30, upon receiving the IP address change notificationfrom the second storage device 200, updates the storage management tableT1, and notifies the host 10 that the IP address of the target has beenchanged (S17).

The host 10, upon receiving the IP address change notification from thename server 30, commences discovery (S18), and acquires the new IPaddress of the target from the name server 30. And, the host 10 rewritesthe IP address set in the access destination information with the IPaddress of the second storage device 200 (S19), switches the accessdestination to the second storage device 200 (S20), and logs onto thesecond storage device 200 (S21). Subsequently, the host 10 will be ableto read and write data to and from the second storage device 200.

FIG. 10 is a flowchart showing the processing in a case when the host 10issues a read request of data to the second storage device 200.

When the host 10 designates LBA or the like and issues a read request(S31), the second storage device 200 refers to the data migration statemanagement table T2 (S32), and determines whether the requested data hasbeen migrated to the second storage device 200 (S33).

When the requested data has been migrated to the second storage device200 (S33: YES), the second storage device 200 reads such data (S34), andtransmits this to the host 10 (S35). The host 10 receives datatransmitted from the second storage device 200 (S36).

Meanwhile, when data requested from the host 10 has not yet beenmigrated to the second storage device 200 (S33: NO), the second storagedevice 200 increments 1 to the value of counter Cx for counting thenumber of accesses to the non-migrated area (S34). The second storagedevice 200 determines whether the counter Cx has reached a prescribedvalue C1 (S38), and, when Cx has not reached C1, reads the requesteddata from the first storage device 100 (S39), and transmits this to thehost 10 (S40).

Meanwhile, when the number of accesses (Cx) by the host 10 to thenon-migrated area has reached the prescribed value C1 (S38: YES), thesecond storage device 200 commences the path switching processing forswitching the access destination of the host 10 to the first storagedevice 100 (S41). The path switching processing will be explained indetail later with reference to FIG. 12.

Incidentally, although it has been explained that the path switchingprocessing is performed when the number of accesses (Cx) by the host 10to the non-migrated area has reached the prescribed value C1, this isnot limited thereto, and, for example, the path switching processing maybe performed when the access to the non-migrated area continues to beover the prescribed value C1. Thereby, for instance, the accessdestination of the host 10 can be switched based on the continuousreadout (sequential access) of video data or the like.

FIG. 11 is a flowchart showing the processing in a case where the host10 requests the second storage device 200 to update the data. When thehost 10 designates LBA or the like and issues a write request (S51), thesecond storage device 200 stores the write data in the cache memory 230(S52), and notifies the host 10 of the completion of the writeprocessing (S53). The host 10 receives the processing completenotification of the write command and ends the processing routine (S54).

The second storage device 200 sets the migration complete flagcorresponding to the write data to “1”, and updates the data migrationstate management table T2 (S55). Next, the second storage device 200issues a write request to the first storage device 100 so as to reflectthe write data received from the host 10 to the volume of the migrationsource (S56).

The first storage device 100 stores the write data received from thesecond storage device 200 in the cache memory 130 (S57), and notifiesthe completion of write processing to the second storage device 200(S58). The first storage device 100 executes the destage processing forwriting the write data in a prescribed disk drive 161 (S59).

When the second storage device 200 completes writing the data in thefirst storage device 100, it executes the destage processing for writingthe write data in a prescribed disk drive 261 (S60).

As described above, when the host 10 writes write data in the secondstorage device 200 of the migration destination, by reflecting thiswrite data also in the first storage device 100 of the migration source,the stored content of the migration source volume and migrationdestination volume can be matched. Therefore, even in a case where thenon-migrated area is accessed frequently and the access destination ofthe host 10 is switched from the second storage device 200 to the firststorage device 100, the conformity of data can be maintained.Incidentally, in FIG. 11, although an example was illustrated in whichthe completion notification of the write processing and the destageprocessing are executed asynchronously, depending on the case, thecompletion notification of the write processing may be conducted afterthe completion of the destage processing.

FIG. 12 is a sequence showing a state of reswitching the accessdestination of the host 10 from the second storage device 200 to thefirst storage device 100. The second storage device 200, as explainedtogether with FIG. 10, checks the frequency of access by the host 10 tothe non-migrated area (S71), and, when access in excess of theprescribed number C1 is confirmed (S72: YES), commences the pathswitching processing.

Foremost, the second storage device 200 acquires its personalconfiguration information (S73), logs onto the first storage device 100(S74), and copies the configuration information to the first storagedevice 100 (S75).

The first storage device 100 personally sets the configurationinformation copied by the second storage device 200 (S76), and notifiesthe name server 30 to the effect that the IP address of the target hasbeen changed (S77). When the name server 30 receives this IP addresschange notification, it updates the storage management table T1, andnotifies the host 10 of the change of IP address (S78).

The host 10 commences discovery upon receiving the notification from thename server 30 (S79), acquires the new IP address of the target from thename server 30, and changes the IP address of the access destination(S80). Thereby, the access destination of the host 10 is switched fromthe second storage device 200 to the first storage device 100 (S81), andthe host 10 logs onto the first storage device 100 (S82) so as to readand write data.

FIG. 13 is a flowchart showing the path switching processing to theexecuted by the first storage device 100. When the host 10 issues a readrequest (S91), the first storage device 100 refers to the data migrationstate management table T2 (S92), and determines whether the requesteddata has been migrated to the second storage device 200 (S93).

When the requested data has not yet been migrated to the second storagedevice 200 (S93: NO), the first storage device 100 reads the requesteddata (S94), and transmits this to the host 10 (S95). The host 10receives data from the first storage device 100, and stores this in alocal memory.

Contrarily, when the data requested from the host 10 has been migratedto the second storage device 200 (S93: YES), the first storage device100 increments 1 to the counter Cy for counting the number of accessesto the migrated area (S97). And, the first storage device 100 determineswhether the number of accesses (Cy) to the migrated area has reached aprescribed value C2 (S98).

When the number of accesses (Cy) to the migrated area has not reachedthe prescribed value C2 (S98: NO), the first storage device 100 readsdata from its own volume and transmits this to the host 10 (S94, S95).

Meanwhile, when the number of accesses (Cy) to the migrated area hasreached the prescribed value C2 (S98: YES), the first storage device 100commences the path switching processing (S99). This path switchingprocessing, as described above, may be performed by changing the IPaddress of the target to the address of the switching destination, andnotifying the name server 30.

As described above, as a result of reswitching the access path switchedto the first storage device 100 back to the second storage device 200,the data migrated to the second storage device 200 can be utilized. And,for example, when the second storage device 200 comprises ahigh-performance disk drive 261 such as an FC disk or a high-capacitycache memory 230 and so on, data can be provided to the host 10 byutilizing the high performance of the second storage device 200, and theresponse of the storage system can be improved thereby.

Further, by switching the access destination of the host 10 between thesecond storage device 200 of the migration destination and the firststorage device 100 of the migration source based on the data migrationstate of the volume (how much data has been migrated) and the accessstate of the host 10 (access in excess of a prescribed frequency to thenon-migrated area or migrated area), the generation of redundant I/Ooperations can be suppressed, and the response of the storage system canbe improved by preventing the frequent switching of the accessdestination.

FIG. 14 is a flowchart showing the path switching processing to beexecuted with the first storage device 100 after the completion of datamigration. The first storage device 100 monitors whether the datamigration is complete (S101), and, when the data migration is complete(S101: YES), it determines whether the access destination of the host 10is the second storage device 200 (S102).

The determination of such current access destination, for example, mayalso be made by determining whether the configuration information of thefirst storage device 100 has been rewritten by the second storage device200.

The first storage device 100 ends the processing routine when thecurrent access destination of the host 10 is the second storage device200 (S102: YES). Contrarily, when the access destination of the host 10is the first storage device 100 (S102: NO), the first storage device 100switches the access destination of the host 10 to the second storagedevice 200, and executed path switching processing (S103). As describedabove, after the completion of data migration, by inspecting the accessdestination of the host 10 and switching it to the second storage device200, after the completion of data migration, data can be provided fromthe second storage device 200 of the data migration destination to thehost 10.

In the present embodiment, the foregoing constitution yields thefollowing effects. In the present embodiment, the access destination ofthe host 10 is switched to the first storage device 100 based on theaccess state to the non-migrated area in which data has not yet beenmigrated from the migration source volume of the first storage device100 to the migration destination volume of the second storage device200.

Therefore, the generation of redundant I/O regarding the non-migrateddata can be suppressed, the response of the storage system can beimproved, and the increase of network traffic can be prevented. Inparticular, for example, as in video distribution or the like, when thehost 10 is to perform information processing with continuous data, it ispossible to prevent the continuous readout of data existing in thenon-migrated area, and the response can be improved thereby.

In the present embodiment, the access destination of the host 10 isswitched from the first storage device 100 to the second storage device200 based on the access state to the migrated area in which data hasbeen migrated to the second storage device 200 among the access requestsfrom the host 10 to the first storage device 100. Therefore, data thathas already been migrated can be provided from the second storage device200 to the host 10, and the second storage device 200 will be able toexhibit its performance. And, in the present embodiment, since thestorage device of the migration source and migration destination can besuitably switched based on the data migration state and the access statedata of the host, deterioration of the response can be prevented whileperforming data migration.

In the present embodiment, when the data migration is complete, theaccess destination of the host 10 is confirmed as whether it is thefirst storage device 100 or the second storage device 200, and, when theaccess destination of the host 10 is the first storage device 100, theaccess destination of the host 10 is switched to the second storagedevice 200. Therefore, after the completion of data migration, data canbe provided from the second storage device 200 to the host 10.

In the present embodiment, the access destination of the host 10 isswitched by employing the discovery based on iSCSI. Therefore, there isno need to provide a special function on the host 10 side for switchingthe access destination, and the response of the storage system can beimproved with a relatively simple constitution.

2. Second Embodiment

The second embodiment is explained with reference to FIG. 15 to FIG. 17.In the present embodiment, the name server 30 is not used. FIG. 15 is ablock diagram showing the overall constitution of the storage systempertaining to this embodiment. In the present embodiment, the point ofthe name server 30 being omitted differs from the first embodiment.

FIG. 16 is a sequence showing the overall processing of data migration.When a user designates the start of migration to the first storagedevice 100 via the management device 20 (S111), the first storage device100 acquires its own configuration information (S112), logs onto thesecond storage device 200 (S113), and copies this configurationinformation to the second storage device 200 (S114). The second storagedevice 200 personally sets this configuration information, andassociates the IP address of the second storage device 200 with thetarget name (S115).

Next, in the present embodiment, the first storage device 100 logs outfrom the host 10 (S116). When the host 10 detects the logout of thefirst storage device 100, it commences discovery (S117), and acquiresthe new IP address of the target from the second storage device 200.And, the host 10 rewrites the IP address set in the access destinationinformation with the IP address of the second storage device 200 (S118),switches the access destination to the second storage device 200 (S119),and logs onto the second storage device 200 (S120).

FIG. 17 is a sequence showing the state of switching the accessdestination of the host 10 from the second storage device 200 to thefirst storage device 100. The second storage device 200 checks thefrequency of access by the host 10 to the non-migrated area (S131), and,when access in excess of a prescribed number C1 is confirmed (S132:YES), it commences path switching processing.

The second storage device 200 acquires its own configuration information(S133), logs onto the first storage device 100 (S134), and copies theconfiguration information to the first storage device 100 (S135).

The first storage device 100 personally sets the configurationinformation copied by the second storage device 200 (S136). Meanwhile,the second storage device 200 logs out from the host 10 (S137).

The host 10, upon the second storage device 200 logging out, commencesdiscovery (S138), acquires the new IP address of the target from thefirst storage device 100, and changes the IP address of the accessdestination (S139). Thereby, the access destination of the host 10 isswitched from the second storage device 200 to the first storage device100 (S140), and the host 10 logs onto the first storage device 100 toread and write data (S141).

As described above, the present invention can also be employed in astorage system that does not comprise the name server 30.

3. Third Embodiment

The third embodiment is explained with reference to FIG. 18. In thepresent embodiment, a plurality of data migration processes is conductedindependently. FIG. 18 is an explanatory diagram showing a frame formatof the schematic constitution of the storage system of this embodiment.

This storage system has one migration source storage device 100, and aplurality of migration destination storage devices 200, 300. The firststorage device 100 comprises a plurality of migration destinationvolume. One migration source volume forms a copy pair with the migrationdestination volume of the second storage device 200, and the othermigration source volume forms a copy pair with the migration destinationvolume of the third storage device 300.

A plurality of data migration processes, for example, can be performedin two ways. One method is to perform each data migration process viaserial processing, and the other method is to perform one data migrationprocess, and, after the completion thereof, to subsequently start theother data migration process. Either method may be adopted.

4. Fourth Embodiment

The fourth embodiment is explained with reference to FIG. 19. In thisembodiment, a switch 40 is used to manage the data migration state andswitch the access destination of the host 10. FIG. 19 is a block diagramshowing the schematic constitution of the storage device pertaining tothis embodiment.

The switch 40, for example, may include a data migration statemanagement unit 41, an access state monitoring unit 42, and a pathcontrol unit 43.

The data migration state management unit 41 acquires and managesinformation relating to the data migration state from the first storagedevice 100 or the second storage device 200. The access state monitoringunit 42 monitors whether the access request from the host 10 is anaccess request to the non-migrated area or an access request to themigrated area. The access state monitoring unit 42 is used to realizethe respective functions of the first switching indication unit andsecond switching indication unit, and, when the access request to thenon-migrated area exceeds the prescribed number Cx, or, when the accessrequest to the migrated area exceeds the prescribed number Cy, itdesignates the switching of the path. The path control unit 43 switchesthe access destination of the host 10 based on the designated from theaccess state monitoring unit 42.

Incidentally, instead of providing the data migration state managementunit 41 to the switch 40, data migration state management unit may beprovided to either the first storage device 100 or the second storagedevice 200. Here, the data migration state management unit provided toeither the storage device 100 or 200 is able to notify the datamigration state in accordance with the inquiry from the access statemonitoring unit 42 of the switch 40.

Further, in the present embodiment, although the name server 30 isprovided, the access destination of the host 10 can be switched withouthaving to go through the name server 30. Therefore, the name server 30may be omitted.

Incidentally, the present invention is not limited to the foregoingembodiments. Those skilled in the art may make various additions ormodifications within the scope of the present invention. For example,those skilled in the art may suitably combine the respective embodimentsdescribed above.

1. A storage system in which a first storage device, a second storagedevice and a host device are mutually connected communicably,comprising: a data migration unit for migrating data stored in a firstvolume of said first storage device to a second volume of said secondstorage device; a data migration state management unit for managing themigration state of data from said first volume to said second volume; afirst access request processing unit for processing the access requestfrom said host device to said second volume; and a first switchingindication unit for switching the access destination of said host deviceto said first storage device based on the access state to thenon-migrated area in which data has not yet been migrated from saidfirst volume to said second volume.
 2. A storage system according toclaim 1, wherein said first access request processing unit is capable ofprocessing the access request from said host device while said datamigration unit is migrating data from said first volume to said secondvolume.
 3. A storage system according to claim 1, wherein said firstaccess request processing unit acquires data from said second volume andprovides this to said host device when the access request from said hostdevice is an access request to a migrated area in which data has beenmigrated to said second volume, and acquires data from said first volumeand provides this to said host device when the access request from saidhost device is an access request to said non-migrated area.
 4. A storagesystem according to claim 1, wherein said first switching indicationunit switches the access destination of said host device to said firststorage device when the frequency of access from said host device tosaid non-migrated area reaches a first prescribed value.
 5. A storagesystem according to claim 1, further comprising: a second access requestprocessing unit for processing the access request from said host deviceto said first volume; and a second switching indication unit forswitching the access destination of said host device from said firststorage device to said second storage device based on the access stateto the migrated area in which data has been migrated to said secondvolume among the access requests from said host device to said firstvolume.
 6. A storage system according to claim 5, wherein, after saiddata migration unit completes the data migration, said second switchingindication unit confirms whether the access destination of said hostdevice is said first storage device or said second storage device, andswitches the access destination of said host device to said secondstorage device when the access destination of said host device is saidfirst storage device.
 7. A storage system according to claim 5, whereinsaid second switching indication unit switches the access destination ofsaid host device to said second storage device when the frequency ofaccess from said host device to said migrated area reaches a secondprescribed value.
 8. A storage system according to claim 1, wherein saidfirst switching indication unit switches the access destination of saidhost device from said second storage device to said first storage deviceby copying the second constitution information of said second storagedevice to said first storage device, rewriting the first constitutioninformation of said first storage device with said second constitutioninformation, and providing a trigger signal for urging the redetectionof the access destination to said host device.
 9. A storage systemaccording to claim 8, further comprising a storage management device fortransmitting said trigger signal to said host device, wherein saidstorage management device transmits said trigger signal to said hostdevice by notifying said storage management device to the effect thatsaid first constitution information of said first storage device hasbeen rewritten with said second constitution information.
 10. A storagesystem according to claim 8, wherein said trigger signal is provided tosaid host device by logically disconnecting the connection between saidsecond storage device and said host device.
 11. A storage systemaccording to claim 1, wherein said first storage device, said secondstorage device and said host device are mutually connected with iSCSI(internet Small Computer System Interface).
 12. A storage systemaccording to claim 1, wherein said first switching indication unit isprovided to said second storage device, and said second switchingindication unit is provided to said first storage device.
 13. A storagesystem according to claim 1, wherein said first storage device, saidsecond storage device and said host device are mutually connected in atwo-way communicable manner via a switch; and said switch is providedwith at least said data migration state management unit and said firstswitching indication unit.
 14. A data migration method for migratingdata from a first storage device to a second storage device in a storagesystem in which the first storage device, the second storage device anda host device are mutually connected communicably, comprising: a firstswitching step of switching the access destination of said host devicefrom said first storage device to said second storage device; a datamigration step of migrating data stored in a first volume of said firststorage device to a second volume of said second storage device; amanagement step of managing the data migration state of data from saidfirst volume to said second volume; an access request processing step ofprocessing the access request from said host device to said secondvolume; a step of detecting the access state to the non-migrated area inwhich data has not yet been migrated from said first volume to saidsecond volume; and a second switching step of switching the accessdestination of said host device from said second storage device to saidfirst storage device based on the access state to said detectednon-migrated area.
 15. A data migration method for a storage systemaccording to claim 14, wherein said second switching step switches theaccess destination of said host device from said second storage deviceto said first storage device when the frequency of access from said hostdevice to said non-migrated area reaches a first prescribed value.
 16. Adata migration method for a storage system according to claim 14,further comprising a third switching step after said second switchingstep, wherein said third switching step reswitches the accessdestination of said host device from said first storage device to saidsecond storage device based on the access state to the migrated area inwhich data has been migrated to said second volume among the accessrequests from said host device to said first volume.
 17. A datamigration method for a storage system according to claim 16, furthercomprising a fourth switching step after said third switching step,wherein, when said data migration is complete, said fourth switchingstep confirms whether the access destination of said host device is saidfirst storage device or said second storage device, and switches theaccess destination of said host device to said second storage devicewhen the access destination of said host device is said first storagedevice.
 18. A data migration method for a storage system according toclaim 14, wherein said second switching step switches the accessdestination of said host device from said first storage device to saidsecond storage device by copying the first constitution information ofsaid first storage device to said second storage device, rewriting thesecond constitution information of said second storage device with saidfirst constitution information, and providing a trigger signal forurging the redetection of the access destination to said host device.19. A data migration method for a storage system according to claim 18,wherein said trigger signal is provided to said host device by logicallydisconnecting the connection between said second storage device and saidhost device.
 20. A storage system in which a first storage device, asecond storage device and a host device are mutually connectedcommunicably via a switch, wherein: (1) said first storage devicecomprises: a first control unit for performing data communication withthe outside and data communication with a first memory device group viasaid switch; and a first volume disposed on a memory area provided bysaid first memory device group; (2) said second storage devicecomprises: a second control unit for performing data communication withthe outside and data communication with a second memory device group viasaid switch; and a second volume disposed on a memory area provided bysaid second memory device group; (3) said name server comprises a namemanagement unit for respectively managing the target name and IP(Internet Protocol) address of said first storage device and said secondstorage device; (4-1) said first storage device and/or said secondstorage device is provided with a data migration unit for migrating datastored in said first volume to said second volume; (4-2) said firststorage device and said second storage device are respectively providedwith a data migration state management unit for managing the migrationstate of data from said first volume to said second volume; (4-3) saidsecond storage device is provided with a first switching indication unitfor switching the access destination of said host device from saidsecond storage device to said first storage device based on the accessstate to the non-migrated area in which data has not yet been migratedfrom said first volume to said second volume; (4-4) said first storagedevice is provided with a second switching indication unit for switchingthe access destination of said host device from said first storagedevice to said second storage device based on the access state to themigrated area in which data has been migrated to said second volumeamong the access requests from said host device to said first volume;(5-1)said first switching indication unit and said second switchingindication unit copy the constitution information (including the devicename, target name (iSCSI name), and LUN (Logical Unit Number) of theaccess target) of the storage device to become the switching source tothe storage device to become the switching destination, and rewrite thetarget name of the storage device to become said switching destination;(5-2) change of IP address corresponding to said target name is notifiedfrom the storage device to become said switching destination to saidname server; and (5-3) said name server urges said host device toperform the discovery of the storage device.